1. Field of the Invention
The invention relates to a method and a related device for the pre-treatment of electrically conductive or non-conducting substrates in a vacuum. The preferred field of application is the treatment of surfaces that require an adhesive coating in a subsequent vacuum coating process. Tools, made of steel, hard metal or ceramic, for example are treated in accordance with the method.
2. Discussion of Background Information
It is known (R. A. Haefer: Oberflachen- und Dunnschicht-Technologie, Vol. I, pp. 30 ff., Springer-Verlag, Berlin 1987) that substrates to be coated in a vacuum have to be exposed to a multi-step treatment process. It usually consists of one or several mechanical or chemical cleaning steps, as well as a subsequent vacuum treatment in the coating facility. Here the interfering surface layers, for example water films or thin oxide layers, are removed, if needed, the nucleation locations for the layer to be deposited are created, or even adhesion-promoting inter-coatings are applied.
Different methods of vacuum pre-treatment have been developed, depending on the electrical conductance of the substrates. Electrically non-conducting substrates, such as glass or oxide ceramics, are often cleaned by a glow discharge, which is ignited between the grounded substrate fastener as the anode and a special glow electrode as the cathode in a working gas, preferably Argon, with a pressure around 10 Pa. The substrate is thereby hit by electrons, which primarily cause the desorption of contaminant layers, in particular of the water film, and cause the substrate to heat up. Condensation nuclei can furthermore be created by the sputtering of the electrode material. The primary disadvantage of this method is based on the low current densities that can be achieved, and thereby its low efficiency. It is furthermore impossible to remove thermally stabile contamination layers, for example made of oxides, with the aid of electron bombardment of the substrate.
Electrically conductive substrates are pre-treated in the simplest manner by using an sputtering process. The discharge required for this is ignited between the negatively biased substrate as the cathode and the grounded vacuum chamber, or an etching electrode, as the anode. The foreign atoms are removed by ion bombardment of the substrate and the surface of the substrate is activated. Additional particles and radiation quanta of the plasma assist the activation by interacting with the substrate surface.
It is known that the effect of the pre-treatment can be increased, depending on the substrate material, by using reducing or oxidizing gas mixtures (DE 31 44 192).
It is furthermore known that the overlay of a magnetic field, in particular in accordance with the magnetron principle, leads to the strengthening of the discharge and thereby to an intensification of the pre-treatment (DD 136 047).
The substrate pre-treatment with a direct current-glow discharge often times exhibits process instabilities. They are caused by the non-conductive areas of the substrate surface, for example isolated areas of oxide, to electrically charge up, and by the sudden release of the glow discharge in a resulting arc discharge. This phenomenon, also referred to as "arcing" does not only impede the removal of the contaminant layers, but also leads to the occurrence of localized damages of the substrate surface.
A known method, which overcomes these disadvantages, is the pre-treatment in a RF-plasma, preferably at a frequency of 13.56 MHz. For non-conductive substrates, a RF-plasma is generally required. With the proper design of the electrode surfaces, a so-called self-bias-voltage occurs, which creates a beam of accelerated ions to the substrate. Numerous elementary processes are thereby triggered, leading to the cleaning of the substrate surface by sputtering and to an activation of the surface. All RF-processes have the disadvantage of exhibiting low efficiency, high energy losses during the electrical adaptation, and an elaborate technical outlay. In particular for substrates with large surfaces, a homogenous substrate pretreatment is practically impossible.
It is also known that a pre-treatment of substrates in vacuum can be achieved by using charged-particle sources. With the aid of electron beam sources, a heating of the substrate and a desorption of volatile adsorption layers is essentially achieved. By bombarding the substrates with ion sources or plasma sources, non-volatile surface layers can also be removed by sputtering (DD 292 028; DE 37 08 717 C2).
The bombardment of the substrate with metal ions (Kadlec et al., Surf. Coat. Technol., 54/55, 1992, 287-296), still results in the creation of a coating mixture of substrate atoms and implanted foreign atoms. The process is therefore also known as "ion mixing". It leads to very good adhesion of the coatings applied subsequently in the vacuum.
All of the stated beam processes for the pre-treatment of substrates have the disadvantage of requiring very elaborate equipment, and are therefore associated with high costs. This is especially true for substrates with large surfaces.
For special applications, for example the pre-treatment of polymer surfaces, the use of microwave-plasmas is known. These do not achieve a removal of contamination layers, but rather the modification of the physical and chemical adhesion properties between the substrate and the coating material. The effectiveness is extremely dependent on the material and tied to the chemical nature of the substrate and the coating material.